Concept 40.1 Excretory Systems Maintain Homeostasis of the Extracellular Fluid

• Excretory systems maintain the volume, concentration, and composition of the extracellular fluids and eliminate nitrogenous wastes. Urine is the output of excretory systems.
   
• Osmolarity is the overall solute concentration of a fluid. Animals must maintain the osmolarity of their extracellular fluids within an acceptable range.
   
• Animals can be osmoconformers or osmoregulators. Terrestrial and freshwater animals must be osmoregulators. Most marine invertebrates are osmoconformers, but most marine vertebrates are osmoregulators. Review Figure 40.1
   
• In addition to regulating the overall osmolarity of the extracellular fluid, most animals also regulate the concentrations of specific ions.
   
• The mammalian kidney reabsorbs bicarbonate ions from the urine and returns them to the blood, where they act as a pH buffer to absorb excess H⁺ ions.
   

Concept 40.2 Excretory Systems Eliminate Nitrogenous Wastes

• Metabolism of proteins and nucleic acids produces toxic nitrogenous wastes, which must be eliminated from the body. The most common nitrogenous waste is ammonia. Review Figure 40.2
   
• Ammonotelic animals produce ammonia as their primary nitrogenous waste. They are typically water-breathing aquatic animals that eliminate ammonia by diffusion across their gill membranes.
   
• Ureotelic animals detoxify ammonia by converting it to urea before excretion, and then flush the urea out with water. These animals include mammals, most amphibians, and cartilaginous fishes.
   
• Uricotelic animals convert ammonia to uric acid. They include insects, some amphibians, and birds and other reptiles. Uric acid can be excreted without much loss of water.
   
• Most species produce more than one kind of nitrogenous waste.
   

Concept 40.3 Excretory Systems Produce Urine by Filtration, Reabsorption, and Secretion

• Excretory systems produce urine through the processes of filtration, reabsorption, and secretion.
   
• Water cannot be actively transported against its concentration gradient, so it must be moved across membranes by a difference in either osmolarity or pressure.
   
• In annelid worms, blood pressure causes filtration of the blood across capillary walls and into the coelom, where it is taken up by metanephridia, which adjust the composition of the filtrate by active transport. Review Figure 40.3 and WEB ACTIVITY 40.1
   
• The Malpighian tubules of insects receive ions and nitrogenous wastes by active transport across the tubule cells. Water follows by osmosis. Ions and water are reabsorbed from the hindgut and rectum, so the insect excretes semisolid wastes. Review Figure 40.4
   
• The functional unit of the vertebrate kidney is the nephron. A nephron consists of a ball of capillaries called a glomerulus, from which the blood is filtered, a renal tubule, which processes the filtrate into urine to be excreted, and a system of peritubular capillaries, which surround the tubule and assist with secretion and reabsorption. The vertebrate kidney is adapted for ridding the body of excess water. Review Figures 40.5 and 40.6 and WEB ACTIVITY 40.2
   
• The rate at which the filtrate flows into Bowman's capsule is called the glomerular filtration rate (GFR).
   
• Several groups of vertebrates have evolved different mechanisms for conserving water. Marine bony fishes produce little urine. Cartilaginous fishes retain urea so that the osmolarity of their body fluids remains close to that of seawater. Most amphibians produce large amounts of dilute urine. Reptiles lose little water in urine because they excrete nitrogenous wastes as uric acid. Mammals must lose some water to flush out urea, but they can produce urine more concentrated than their extracellular fluids.
   

Concept 40.4 The Mammalian Kidney Produces Concentrated Urine

• The glomeruli, Bowman's capsules, and proximal and distal convoluted tubules are located in the cortex of the kidney. Straight sections of renal tubules called loops of Henle and collecting ducts are arranged in parallel in the medulla of the kidney. Review Figure 40.7 and WEB ACTIVITY 40.3
   
• The proximal convoluted tubule reabsorbs water, NaCl, and certain solutes, reducing the renal filtrate's volume without appreciably changing its osmolarity.
   
• The loops of Henle create a concentration gradient in the interstitial fluid of the renal medulla by a countercurrent multiplier mechanism. Urine flowing down the collecting ducts is concentrated by the osmotic reabsorption of water caused by the concentration gradient in the surrounding interstitial fluid. Review Figure 40.8, ANIMATED TUTORIAL 40.1, and WORKING WITH DATA 40.1
   

Concept 40.5 The Kidney Is Regulated to Maintain Blood Pressure, Blood Volume, and Blood Composition

• Kidney function in mammals is controlled by autoregulatory mechanisms that maintain a constant high GFR even if blood pressure varies.
   
• The kidneys release renin if GFR falls. Renin activates angiotensin, which causes the constriction of efferent glomerular arterioles and peripheral blood vessels, causes the release of aldosterone (which enhances water reabsorption), and stimulates thirst. Review Figure 40.9 and INTERACTIVE TUTORIAL 40.1
   
• A fall in blood pressure or a rise in blood osmolarity increases the release of antidiuretic hormone (ADH) from the posterior pituitary. ADH increases the permeability of the collecting duct to water and therefore increases the reabsorption of water from the urine.
   
• If the volume of blood returning to the heart increases and stretches the atrial walls, heart cells release atrial natriuretic peptide (ANP). ANP increases excretion of salt and water, reducing blood volume and blood pressure.
   

WEB ACTIVITY 40.4 for a review of the major human organ systems.